1. Field of the Invention
The present invention relates generally to improvements in power supplies and, more particularly, to power supplies that utilize switched capacitors to generate negative output voltages from positive voltage sources.
2. Brief Description of the Prior Art
Inverting power supplies that generate negative voltages from a positive power source are used for a variety of purposes and it has long been known that a negative voltage can be generated using a capacitor that is switched between a power source, that provides a positive voltage with respect a system ground, and an output terminal that is to be maintained at a negative voltage with respect to the system ground. However, it has been found that conventional inverting power supplies constructed using this approach have shortcomings that limit their use in particular applications or require undesirable tradeoffs in adapting them to particular applications.
A problem that has arisen in the use of magneto-resistive read heads in disc drives that store computer files has highlighted these shortcomings. A magneto-resistive head senses magnetic fields adjacent surfaces of rotating discs of the disc drives by measuring the voltage necessary to maintain a constant current through a magneto-resistive element of the head. The magnetic fields are produced by flux transitions, corresponding to logical 1's in data stored by the disc drive, along data tracks in magnetizable surface coatings formed on the discs. Thus, by detecting peaks in the magnetic field adjacent a disc surface, the stored data can be recovered.
The problem that has arisen with the use of these heads in a disc drive is that maintaining a current through the magneto-resistive element can result in a build-up of electrostatic charge on the head and such electrostatic charge can result in arcing between the head and the disc surface. The arcing vaporizes small amounts of the surface coating and the vaporized material can collect on the heads to interfere with the reading of data from the discs.
The electrostatic charge build-up arises from the combination of the relative motion between the disc surfaces and head and the use of power supplied by a computer with which the disc drive is used. This power, usually supplied at 5 volts with respect to a common ground for both the computer and the disc drive places the magneto-resistive head at a positive voltage with respect to the discs which are commonly grounded. Hence, because of the relative motion between the discs and the head, a charge can be built up on the head in much the same manner that a charge is built up in a Van de Graaf generator or a Wimhurst machine.
The problem presented by the build up of electrostatic charge on the heads of discs drives of this type can be overcome by providing the disc drive with an inverting power supply that produces a voltage that is negative with respect to the system ground, and returning the current through the magneto-resistive element to a negative output terminal at which this negative voltage appears. Thus, the magneto-resistive element can be maintained at substantially ground potential to prevent the heads from becoming charged.
Unfortunately, as noted above, conventional inverting power supplies have shortcomings which, though not so severe as to make them unusable, limit their value in compensating for the problem described above. Thus, for example, on a time scale corresponding to the passage of flux transitions on a data track by a read head, voltage regulation in conventional inverting power supplies takes place slowly so that some charge build up can occur despite the use of the inverting power supply. Moreover, ripple in the negative voltage supplied by the inverting power supply has the effect of introducing noise into the disc drive read channel, of which the magneto-resistive element is a part, and such noise can generate errors in the reading of a computer file from a disc. While these problems can be compensated by using large reservoir capacitors between the negative output terminal of the inverting power supply and the system ground, the use of large, and consequently bulky, capacitors in a disc drive is contrary to another desirable characteristic of a disc drive; specifically, small size that will permit a disc drive to take up little space in the case of a computer with which a disc drive might be used. Such size can be an important consideration in; for example, a lap top computer. Thus, the shortcomings of conventional inverting power supplies require a tradeoff between the extent to which the arcing problem is overcome and other desirable disc drive characteristics. Consequently, while conventional inverting power supplies can be used to minimize the arcing problem, they exact a price that limits their usefulness for this purpose.